The present invention relates to a method and to an installation for chemically purifying vapor in a dehydrator with mechanical vapor compression.
A dehydrator with mechanical vapor compression is a dehydrator having a heat exchange wall with one face adapted to receive matter to be dehydrated, which matter is deposited on said face in the form of a thin layer.
This hot face is inside an xe2x80x9cevaporationxe2x80x9d chamber.
The matter deposited on the hot face is dehydrated, with the water and other volatile substances it contains evaporating into the evaporation chamber.
The vapor is taken from this chamber and it is compressed mechanically by means of a compressor, after which it is introduced into a second chamber referred to as a xe2x80x9ccondensationxe2x80x9d chamber and containing the opposite face of the heat exchange wall.
By means of this arrangement, the quantity of heat which is given off by vapor condensing in the condensation chamber is transferred to the matter by being conducted through the heat exchange wall to the other face of the disk, where said quantity of heat serves to evaporate an equivalent volume of liquid that is to be found in the layer of matter to be dehydrated that is spread on said face.
Thus, the heat given off by condensation is recovered for evaporation, thus making it possible to work with energy input that is very low, corresponding more or less to the mechanical energy required for compressing the vapor.
This type of dehydrator is particularly well adapted to treating farmyard manure, and in particular pig manure; working pressure and temperature in the evaporation enclosure are then about 1 bar and about 100xc2x0 C., while in the condensation chamber they are about 1.4 bars and about The heat exchange wall is generally a moving wall following a cyclical path, the wet matter being deposited on the hot face at the start of a cycle and the dry residue being scraped off the hot face at the end of the cycle.
In a known type of dehydrator with mechanical vapor compression as described in U.S. Pat. No. 5,810,975, the heat exchange walls are the top walls of a stack of horizontal hollow disks mounted coaxially on a vertical axis tubular shaft which is rotated continuously at uniform speed.
By way of indication, the number of disks in the stack is thirty, for example; each disk has a diameter of about 2 meters (m), and rotates at a relatively slow speed, about 0.33 revolutions per minute (rpm).
The set of disks is placed inside a vessel which constitutes the evaporation chamber; the condensation chamber is constituted by the space inside the disks and the tubular shaft, with the inside of the shaft being in communication with each of the disks.
The present invention is particularly appropriate for chemically purifying the vapor which is produced in the evaporation chamber of such a dehydrator.
It is also particularly adapted to treating farmyard manure, and in particular pig manure, where treatment must be performed at extremely low cost price and correspondingly must make use of solutions in which energy consumption is extremely small.
For a manure treatment method to be commercially acceptable, the energy required for treatment must be less than about 50 kilowatt-hours per cubic meter (kWh/m3) of manure, and preferably less than 30 kWh/m3.
To achieve this objective, it is necessary to ensure that the vapor condensation mechanisms in the condensation chamber (inside the disks when using a dehydrator of the type mentioned above) are not impeded by the presence of gas that is not condensable or of substances having a condensation temperature that is lower than that of water.
In various kinds of matter for dehydration, and in particular pig manure, the volatile substances that evaporate together with the water during dehydration represent a significant mass, of about 15 kilograms per cubic meter (kg/m3) of manure to 25 kg/m3.
This vapor contains numerous substances that are very different from one another, in particular carbon dioxide (CO2) gas, ammonia (NH3), various volatile fatty acids (VFAs) in the [CH3(CH2)nCCOH] family, ranging from acetic acid [CH3COOH] to capric acid [CH3(CH2)8COOH], phenols, and solvents such as alcohol, ethers, ketones, aldehydes, etc.
Some of these vapors are not condensable under the temperature and pressure conditions implemented (1 bar, 100xc2x0 C); this applies to the carbon dioxide gas, the ammonia, any air, and methane and hydrogen if there has been any fermentation.
Amongst the vapors that are difficult to condense, there are the solvents, the volatile fatty acids (VFAs), and the phenols.
To obtain good operation of the dehydrator, it is necessary to begin by eliminating those undesirable vapors to as great as extent as possible, and then to ensure that the residual substances are not harmful.
Consequently, the matter to be dehydrated is subjected to prior purification.
Thus, for example, the matter to be dehydrated passes initially through a heat exchanger and degasifier device which pre-warms it to a temperature of about 85xc2x0 C., for example.
Matter such as pig manure often contains ammonium carbonates which dissociate above 50xc2x0 C. into carbon dioxide (CO2) and ammonia (NH3), such that during pre-warming in the heat exchanger, the matter loses a large fraction of its carbon dioxide together with water vapor and other unwanted gases; in contrast, nearly all the ammonia remains in solution in the matter.
Thereafter, it is advantageous to allow the matter to stand for a few hours in heated degassing and defoaming vessels, so as to bring its temperature up to about 98xc2x0 C., immediately prior to applying the matter to the hot face of the dehydrator.
Thus, when it reaches the dehydrator, the matter has already lost 75% to 90% of its carbon dioxide, and the above pretreatment is particularly advantageous, economically speaking.
The object of the present invention is to use chemical purification of the vapor generated in the evaporation chamber during dehydration treatment in order to eliminate all, or nearly all, of the above-mentioned undesirable substances, in particular carbon dioxide, ammonia, VFAs, phenols, and other solvents, . . .
To this end, the method of the present invention is remarkable in that the vapor is taken from the evaporation chamber of the dehydrator and, prior to being compressed and at a temperature substantially equal to 100xc2x0 C., it is caused to pass in succession through a series of purifiers, and in each of the purifiers it is forced through rain containing a purifying solution, at least one of the rain solutions being water, another containing an acid, and another containing a base.
According to other possible characteristics of the method:
the acid is a solution comprising sulfuric acid and nitric acid;
the base is a solution of lime, of potassium hydroxide, or of sodium hydroxide; and
the rain is generated by means of a rotary tube pierced by a multitude of small holes, the inside of the tube containing the solution, said solution being sprayed centrifugally in the form of fine droplets to the outside of the tube.
The installation is remarkable in that it comprises a set of purifiers connected in series one after another, means for bringing the vapor to be purified to the inlet of said set, and means for feeding the treated vapor to the compressor forming part of the dehydrator, each of the purifiers being in the form of a box within which there is generated a rain of droplets of a purifying solution through which the vapor is constrained to pass, which vapor is raised to a temperature substantially equal to 100xc2x0 C., and in that at least one of the rain solutions is water, at least one other contains an acid, and at least a third contains a base.
Furthermore, according to certain possible additional but non-limiting characteristics of the invention:
said set comprises at least three purifier banks, each comprising a plurality of purifiers, one of the banks treating the vapor with water, another treating it with acid, and another treating it with a base;
each of said banks is connected to a distinct vessel for recovering the solutions that have been used for treatment purposes;
the installation forms part of a dehydrator whose condensation chamber is constituted by a stack of intercommunicating horizontal hollow disks carried by a central rotary tubular shaft having a vertical axis, said stack being installed inside a vessel constituting the evaporation chamber, the matter to be dehydrated being applied in the form of a thin layer onto the top faces of the disks, and inside each disk there is received a horizontal disk-shaped partition fixed to the central shaft, with the diameter of the partition being smaller than that of the hollow disk, said partition being spaced apart from both horizontal faces of the disk and constituting a baffle for directing vapor flow;
the vapor delivered by the compressor is brought to the inside of said tubular shaft, the shaft communicating via its top end with the top disk, above its partition, such that the vapor travels downwards through the stack of disks;
the vapor carrying non-condensable gas is taken from the bottom disk of the stack;
it includes a system for recovering heat from the vapor conveying non-condensable gas;
said heat recovery system comprises a coil immersed in a trough of water disposed inside the evaporation chamber, with the vapor conveying non-condensable gas being caused to flow in the coil;
the installation includes a burner suitable for burning off the non-condensable gas at its outlet; and
the installation includes means for delivering the condensation water presence in the vapor at the outlet from said coil into the trough.